public class Solution {
    public int longestPalindromeSubseq(String s) {
        int len = s.length();
        char[] ch = s.toCharArray();

        int[][] dp = new int[len][len];

        int ret = 0;
        for (int i = len - 1; i >= 0; i--) {//从下往上
            for (int j = i; j < len; j++) {//从左往右
                if (ch[i] == ch[j]) {
                    if (i == j) {
                        dp[i][j] = 1;
                    } else if (i + 1 == j) {
                        dp[i][j] = 2;
                    } else {
                        dp[i][j] = dp[i + 1][j - 1] + 2;
                    }
                } else {
                    dp[i][j] = Math.max(dp[i][j - 1], dp[i + 1][j]);
                }
                ret = Math.max(ret, dp[i][j]);
            }
        }
        return ret;

    }
}
